1. Field of the Invention
The present invention relates to a catalyst for removing nitrogen oxides from a gas containing the same, and a method of using the catalyst. More particularly, it relates to a catalyst for catalytically decomposing nitrogen oxides, and a method for catalytically decomposing nitrogen oxides by using the catalyst, which suffers little deterioration even when coexisting with oxygen or sulfur oxides.
2. Description of the Related Art
Nitrogen oxides in combustion waste gases discharged from, for example, industrial plants and automobiles, are major causes of photochemical smog, and the development of methods of eliminating these substances is a crucial and urgent social need from the standpoint of environmental protection. Among the nitrogen oxides, nitric monoxide (NO) is particularly difficult to remove, and various methods of accomplishing this have been investigated. For example, the catalytic reduction method is considered effective, and rapid progress has been made in the development of this method. Nevertheless, this method requires the use of a reducing agent such as ammonia, hydrogen or carbon monoxide, and further, requires special equipment for the recovery or decomposition of any unreacted reducing agent. In contrast, the catalytic decomposition method does not require a special additive such as the reducing agent and can decompose and separate a nitrogen oxide into nitrogen and oxygen merely by passing it through a catalyst bed. Further, the process is simple, and therefore, this is the most preferable method. According to previous research, a NO decomposition activity was observed in Pt and CuO, Co.sub.3 O.sub.4, among others, but because all of these substances are poisoned by oxygen, which is the decomposition product, they are not practically useful as catalysts.
Accordingly, the present inventors carried out intensive studies on catalysts useful for catalytic decomposition, and found that several kinds of zeolites with exchanged copper ions exhibit a stable decomposition activity. Further, the inventors found that a zeolite containing copper ions and having a specific crystal structure not only exhibits an extremely stable and high activity as a catalytic decomposition catalyst for NO, but also maintains that activity at the same level even when coexisting with a sulfur oxide (see Japanese Unexamined Patent Publication (Kokai) No. 60-125250).
Nevertheless, the above-mentioned catalyst does not have a sufficient activity or sufficient heat resistance at high temperatures, and thus can not as yet be put to practical use.